A mixture of inorganic compounds which has been fused by heating at a high temperature is known as an electrolyte material for fuel cells, etc. However, molten salts obtained from mixtures of inorganic compounds have relatively high melting points, and even those which are said to be low-melting salts generally have melting points around 300.degree. C. A subject for the utilization of such molten salts in batteries for practical use has been to drastically lower the melting points thereof.
Attempts were hence made to produce an electrolyte having a sufficient ionic conductivity even at room temperature. As a result, room-temperature molten salts obtained by adding an inorganic salt to a salt of an organic compound have been developed.
For example, an electrolyte comprising a mixture of an aluminum halide and either a 1,3-dialkylimidazole halide or a 1,2,3-trialkylimidazole halide is disclosed in JP-A-60-133669 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-60-133670, JP-A-60-136180, JP-B-8-34770 (The term "JP-B" as used herein means an "examined Japanese patent publication"), and The Society of Polymer Science, Japan, Polymer Preprints. Japan, Vol. 46, No. 3 (1997).
At room temperature, the electrolyte comprising those compounds is in a molten state at room temperature and shows high ionic conductivity. However, this prior art electrolyte has drawbacks that the aluminum halide decomposes by the inclusion of even a slight amount of water, and that the phase state of the molten salt is unstable to temperature fluctuations.